Quality Magazine

Other Dimensions: The Problem With Pins

January 28, 2006
Like many instruments in measurement, gage pins are misused.

There are few inspection departments that don't have one or more sets of what are commonly known as gage pins. Ranging in size up to 1 inch or 25 millimeters to about 2 inches or 50 millimeters long, they are very handy for a wide range of measuring situations. But, like many instruments in measurement, they also are misused.

There are few inspection departments that don't have one or more sets of what are commonly known as gage pins. Ranging in size up to 1 inch or 25 millimeters to about 2 inches or 50 millimeters long, they are very handy for a wide range of measuring situations. But, like many instruments in measurement, they also are misused.

In the days before coordinate measuring machines, these sets were used to convert an internal surface, such as a hole, into an external one. When checking hole center distances, these external surfaces easily were measured using equipment such as height gages. In addition, they often were used to identify a hole rather than qualify one as is often done today.

With these applications in mind, their tolerance of either ±0.0002 inch was more than adequate for the task at hand. In addition, their usual length of 2 inches compared to regular reversible gage members was an added bonus. The downside of this has always been the smaller sizes that are easily bent.

Two major problems have plagued users of these types of pins. The first problem is keeping them in the sets they belong to, and the second is calibrating them. The latter problem was and is a cost problem. You can buy new pins cheaper than you can calibrate used ones.

The reason for this imbalance is in the manufacture of the pins. During manufacture, the maker will produce a large quantity of any given size so one measuring setup is amortized to a very low cost on a per pin basis. This is assuming that they check every one in the batch and there is reason to doubt this is always done.

When it comes to calibration, maximum accuracy requires a lot of gage block build-ups to cover the many sizes in a set; this takes time and costs money. You may be fortunate enough to have some good long-range measuring equipment that doesn't require masters every few sizes, but it you figure the overhead cost of using such devices you may find that the overall cost is still expensive relative to the replacement cost. Just so there's no misunderstanding, "good" long-range measurement equipment does not mean blazing away with your 0- to 1-inch digital micrometer.

Gage pins made to the U.S. standard can be out of tolerance for up to a 1/4-inch in from each end of the pin meaning they are not suitable for use as go/no-go gages. The price tells the difference. An equivalent tolerance reversible plug gage member can be up to four times the price of these pins when coming from the same maker. Roundness variations can vary significantly between the makers-not all of which make their products to any published specification.

Economics aside, we are left with the two of the same basic problems: keeping sets together so any calibration done on them carries meaning, and how to reduce calibration costs.

One way to keep all of the pins that belong together in a set is to color code them with a dab of paint on the end face of the pins. A quick glance at the set tells you if there is a pin that belongs to another set. You could use labels as well.

The problem of calibration cost is another matter, but there are ways to reduce it. An easy way to solve this problem is to calibrate only those pins that have been used since they were last calibrated. The secret is in knowing which ones fall into this category.

One method uses stickers placed at the ends of each pin. If the stickers are missing, you assume the pin has been used and it becomes a candidate for calibration. For this to work, the case for the pins must have room to allow for the stickers, or if you prefer, colored tape also can be used. You also might try colored shrink or wrap tubing used in the electronics industry.

An even better way is what I call the "cosmetic compromise" as it covers both situations. You put a dab of nail polish on the ends of each pin and because you have a wide range of color choices, the process tells you what set the pin belongs to and whether or not it has been used. I prefer shocking pink just to add a little zip to the daydreams.

Don't forget to keep a bottle of nail polish remover handy in case you get sloppy. You'll avoid your significant other questioning you about the presence and the weird colors of nail polish on your hands or clothes.

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